AFTER THE ATTACKS: THE ALLIANCE; For First Time, NATO Invokes Joint Defense Pact With U.S.

NATO invoked a mutual defense clause in its founding treaty for the first time today, strongly suggesting that the United States would have the support of the allies if it takes military action against those responsible for attacks on the World Trade Center and the Pentagon.

A NATO statement issued after a meeting of ambassadors to the 19-member alliance said, ''If it is determined that this attack was directed from abroad against the United States, it shall be regarded as an action covered by Article 5 of the Washington Treaty.''

Article 5, the cornerstone of the alliance, says ''an armed attack'' against any of the allies in Europe or North America ''shall be considered an attack against them all.''

It commits NATO members to take the necessary measures, including the use of force, to restore security.

The statement amounted to a powerful expression of European solidarity with the United States after a period in which trans-Atlantic relations have been strained by tensions over the Bush administration's policies in areas ranging from missile defense to the environment.

NATO's secretary general, Lord Robertson, said the declaration did not necessarily mean NATO would get involved in military action. Nor did it mean that Washington was obliged to act through the group.

''At the moment this is an act of solidarity,'' he said. ''It's a reaffirmation of a solemn treaty commitment which these countries have entered into.''

Asked whether he believed the allies would take joint action, Lord Robertson added: ''The country attacked has to make the decisions, it has to be the one that asks for help. The United States is still assessing the evidence available. They are the one to make that judgment.''

In Washington, Secretary of State Colin L. Powell said the statement would ''tee up'' possible collective military action by NATO once the terrorists and those behind them had been identified.

NATO also made clear for the first time that it was prepared to see some acts of terrorism as acts of war, even if such circumstances were not envisioned when the treaty was written in 1949.

''The commitment to collective self-defence embodied in the Washington Treaty was entered into in circumstances very different from those that exist now,'' said the statement adopted by the North Atlantic Council.

''But it remains no less valid and no less essential today, in a world subject to the scourge of international terrorism.''

The statement also noted pointedly that when NATO leaders met in 1999 on the organization's 50th anniversary, they condemned terrorism as a threat to world peace and affirmed their ''determination to combat it in accordance with their commitments to one another.''

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Any decision to embark on joint military action would require further deliberation, as would a decision to place national forces under joint command.

But diplomats said the resolution was a potent gesture of political support for the United States even if it should it decide to act on its own.

NATO officials said the United States had not asked for the statement, but had said they would welcome it.

As the United States is the dominant power in the organization, it appeared certain that the administration had played a central role in the adoption of the resolution.

The four-paragraph resolution, passed unanimously, said ''The United States' NATO allies stand ready to provide the assistance that may be required as a consequence of these acts of barbarism.''

But some European leaders also urged caution. The Swedish and German foreign minister, Anna Lindh and Joschka Fischer, both suggested that it was too early to talk of military action when so little was known about the origins of the attacks.

Throughout the day, many Europeans continued to express outrage.

In London, Prime Minister Tony Blair recalled Parliament from its recess a month early, saying the voices of democracy must speak out after devastating terror strikes in the United States.

''This was an attack not just on a number of buildings in the United States of America, but on the very notion of democracy,'' he said at a news conference at his office at 10 Downing Street.

While NATO officials met, the foreign ministers of the European Union also gathered in a show of support for the United States. In a statement, the ministers said they would ''spare no efforts to help identify, bring to justice and punish those responsible.''

The foreign ministers declared that Friday would be a day of mourning in all 15 member nations and asked that all Europeans observe three minutes of silence on at noon (6 a.m. Eastern Daylight Time).

''We were all victims of this attack,'' said Belgium's foreign minister, Louis Michel, chairman of the meeting of the European Union.

In an exceptional move, Lord Robertson attended the meeting. ''We have to stand together,'' he said. ''We are two organizations that speak with one voice, one strong voice, that will not stand for terrorism.''

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A version of this article appears in print on September 13, 2001, on Page A00017 of the National edition with the headline: AFTER THE ATTACKS: THE ALLIANCE; For First Time, NATO Invokes Joint Defense Pact With U.S. Order Reprints|Today's Paper|Subscribe